The present invention is directed to an apparatus and methods for imaging using modulated light.
The quality of images obtained using, and operational usefulness of, many scientific and medical imaging systems, and in particular optical or laser based imaging systems, can be significantly degraded by various instrumental or ambient noises or interferences, such as those caused by stray light, electronic noise, or other optical or electronic interference. Such noises include continuous or semi-continuous background or noise sources, such as interferences from noise sources such as room light or electronic noise at specific frequencies, for example from a nearby electric motor, and broadband or xe2x80x9cpink noisexe2x80x9d sources. The measurement environment, along with the electronics and other devices used for any measurement, contributes broadband noise, sometimes called pink noise, into any measurement. This noise can be particularly severe at low frequency, ƒ, due to the well known 1/ƒ properties of such noise.
U.S. Ser. No. 09/072,962, now U.S. Pat. No. 6,096,036, teaches that at least some of these noises or interferences can, in some of these imaging systems and under certain conditions, be significantly reduced through use of certain modulation methods. However, more generally applicable noise reduction methods would allow extension of this modulation concept to other systems and other applications.
Further, while the use of various modulation methods for reduction of interference to improve detection performance for measurements has been generally done in other fields, the present inventors are unaware of the use of modulation and demodulation, as in the present invention, in the various fields of optical imaging, and in particular for use in various optical or laser based systems for scientific and medical imaging. Hence, it is an objective of the present invention to use such methods and apparatus in such fields to overcome the problems caused by such noises and interferences.
The present invention relates to and is directed toward methods and apparatus using a modulated light source and associated circuitry and modulation schemes to substantially improve detection limits in optical imaging and to substantially improve the performance of various optical imaging systems. One way that such methods and apparatus achieve such improvement is by moving the detected signal to a quiescent part of the electromagnetic spectrum. This approach results in suppression of most noise from continuous or semi-continuous background or noise sources and also causes minimization of the effects of pink noise. Modulation methods applicable with the present invention include amplitude modulation, phase modulation, and frequency modulation.
In an illustrative embodiment, the present invention is directed to an apparatus comprising a modulated light source, a modulation frequency reference, a detector, and a demodulator. This apparatus may function independently of or in conjunction with an imaging system, such as for example a microscope or other imaging devices as explained infra, and a data acquisition system. The demodulator may comprise, for example, a lock-in amplifier, a dual-phase lock-in amplifier, a heterodyne or superheterodyne demodulator, a quadrature heterodyne or quadrature superheterodyne demodulator, or similar phase-sensitive demodulation unit.
In a further embodiment, the modulated light source comprises a light source emitting an inherently modulated output. Alternatively, the modulated light source may comprise a separate optical modulator and a continuous wave, modulated, or pulsed light source configured so as to impose a modulation in the output of the light source. This modulated light source, modulation frequency reference, detector, and demodulator may function in conjunction with an imaging system and a data acquisition system or other similar hardware to enable collection of imaging data or multi-dimensional images of a sample.
The present invention is also directed to methods using modulation to substantially improve detection limits in optical imaging, and the performance of various optical imaging systems. Some of these methods use the apparatus described above.
The present invention is also directed to a method for imaging a material, the method comprising the steps of:
encoding light from a light source with a modulation pattern to produce a modulated light;
directing said modulated light onto or into said material, said material emitting a modulated optical signal which is characteristic of the material;
detecting said emitted modulated optical signal from the material and converting the optical signal into a modulated electronic signal which is characteristic of the material; and
demodulating the modulated electronic signal which is characteristic of the material
The present invention can be used in a number of fields, including: various single-photon, two-photon and multiphoton excited microscopes and optical imaging devices and imaging systems based on transmission, absorption, reflection, scatter or luminescence based phenomena; laser scanning microscopes; confocal microscopes; optical coherence tomography systems; terahertz imaging systems; and various scanning probe optical microscopes. This list of fields, however, is merely illustrative, as one skilled in the art upon reading the present application would understand that other fields are also contemplated for the present invention.